fix: Fix sync segments failed due to OOM check in segment loader

[weiliu1031]

issue: #41984
Fix the issue where segment loading fails unnecessarily due to incorrect memory and disk usage checks. Changes include:

• Only perform OOM check when there's actual memory/disk cost during segment loading
• Skip checks when setting segments to delegator doesn't consume resources
• Rename variables for better readability (maxSegmentSize -> maxSegmentMemSize)
• Enhance test cases to cover more scenarios with memory and disk usage settings

[sre-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: weiliu1031
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[mergify]

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[codecov]

Codecov Report

Attention: Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.45%. Comparing base (252d49d) to head (83920e2).
Report is 43 commits behind head on master.

Files with missing lines	Patch %	Lines
internal/querynodev2/segments/segment_loader.go	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #41985      +/-   ##
==========================================
- Coverage   81.67%   80.45%   -1.22%     
==========================================
  Files        1202     1537     +335     
  Lines      185973   216703   +30730     
==========================================
+ Hits       151886   174358   +22472     
- Misses      27803    36053    +8250     
- Partials     6284     6292       +8     

Components	Coverage Δ
Client	79.36% <ø> (ø)
Core	73.10% <ø> (∅)
Go	81.90% <90.90%> (+<0.01%)	⬆️

Files with missing lines	Coverage Δ
internal/querynodev2/segments/segment_loader.go	69.51% <90.90%> (-0.09%)	⬇️

... and 370 files with indirect coverage changes

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[weiliu1031]

sync segment may cost a few memory by delta log, so this pr won't fix the issue